1. Field of the Invention
The present invention is directed to a thermal ink jet printhead. More specifically, the present invention is directed to a thermal ink jet printhead fabrication process utilizing orientation dependent etching (ODE).
2. Background
Thermal ink jet printing is a type of drop-on-demand ink jet systems, wherein an ink jet printhead expels ink droplets on demand by the selective application of electrical pulses to thermal energy generators, usually resistors, located one each in capillary-filled, parallel ink channels a predetermined distance upstream from the channel nozzles or orifices. The channel end opposite the nozzles are in communication with a small ink reservoir to which a larger external ink supply is connected.
U.S. Pat. Reissue No. 32,572 to Hawkins et al. discloses a thermal ink jet printhead and several fabrication processes therefor. Each printhead is composed of two parts aligned and bonded together. One part is a substantially flat substrate that contains on the surface thereof a linear array of heating elements and addressing electrodes, and the second part is a silicon substrate having at least one recess anisotropically etched therein to serve as an ink supply reservoir when the two parts are bonded together. A linear array of parallel grooves are also formed in the second part, so that one end of the grooves communicate with the reservoir recess and the other end of the grooves are open for use as ink droplet expelling nozzles. Many printheads can be made simultaneously by producing a plurality of sets of heating element arrays with their addressing electrodes on a silicon wafer and by placing alignment marks thereon at predetermined locations. A corresponding plurality of sets of channel grooves and associated reservoir are produced in a second silicon wafer. In one embodiment, alignment openings are etched in the second silicon wafer at predetermined locations. The two wafers are aligned via the alignment openings and alignment marks, then bonded together and diced into many separate printheads.
U.S. Pat. No. 4,638,337 to Torpey et al. discloses an improved thermal ink jet printhead similar to that of Hawkins et al., but has each of its heating elements located in a recess. The recess walls containing the heating elements prevent the lateral movement of the bubbles through the nozzle and therefore the sudden release of vaporized ink to the atmosphere, known as blow-out, which causes ingestion of air and interrupts the printhead operation when this event occurs. In this patent a thick film organic structure such as Riston.RTM. or Vacrel.RTM. is interposed between the heater plate and the channel plate. The purpose of this layer is to have recesses formed therein directly above the heating elements to contain the bubble, thus eliminating the occurrence of vapor blow-out and concomitant air ingestion.
U.S. Pat. No. 4,774,530 to Hawkins discloses the use of a patterned thick film insulative layer to provide the flow path between the ink channels and the reservoir, thereby eliminating the fabrication steps required to open the channel groove closed ends to the reservoir recess, so that the printhead fabrication process is simplified.
U.S. Pat. No. 4,786,357 to Campanelli et al. discloses the use of a patterned thick film insulative layer between mated and bonded substrates. One substrate has a plurality of heating element arrays and addressing electrodes formed on the surface thereof and the other being a silicon wafer having a plurality of etched reservoirs, with each reservoir having a set of ink channels. The patterned thick film layer provides a clearance space above each set of bonding pads of the addressing electrodes to enable the removal of the unwanted silicon material of the wafer by dicing without the need for etched recesses therein. The individual printheads are produced subsequently by dicing the assembled substrates.
As disclosed in the above-discussed patents, thermal ink jet printheads are fabricated from two substrates. One substrate contains the heating element and the other contains ink reservoirs/channels. When these two substrates are aligned and bonded together, the reservoirs/channels serve as ink passageways. A plurality of printhead components are formed on separate substrates, so that the substrate pairs may be aligned, mated, and diced into many individual printheads. The substrate for the plurality of sets of reservoir/channels is silicon and the features are formed by an anisotropic etching process. The anisotropic or orientation dependent etching (ODE) has been shown to be a high yielding fabrication process for precise, miniature channel plates. Such printheads are usually about 1/4" to 1" wide and print small swaths of information while being translated across a stationary recording medium such as paper. The paper is then stepped the distance of one swath and the printing process continued until the entire page of paper is printed. This is a low speed process.
U.S. Pat. No. 4,774,530 to Hawkins discloses a two-part ink jet printhead comprising a mated channel plate and a heater plate, which sandwiches a thick film insulative layer that was previously deposited on the heater plate and patterned to provide an ink bypass recess for ink flow from the reservoir to the channels and recesses or pits over each heating element for placement of the heating elements in pits to prevent the vapor bubbles from blowing out the nozzles and causing ingestion of air. This is a typical ink jet printhead configuration and is discussed later with respect to FIG. 2.
U.S. Pat. No. 4,863,560 to Hawkins discloses a three dimensional silicon structure, such as an ink jet printhead, fabricated from (100) silicon wafers by a single side, multiple step ODE etching process. All etching masks are formed sequentially prior to the initiation of etching, with the coarsest mask formed last and etched first. Once the coarse anisotropic etching is completed, the coarse etch mask is removed and the anisotropic etching is resumed on the finer features.
U.S. Pat. No. 5,096,535 to Hawkins et al. discloses the fabrication of a printhead, wherein each of the ink channels is formed by segmenting the channel mask into a series of closely adjacent vias, such that during the subsequent anisotropic etching of the silicon wafer, the thin walls between the segments are eroded away before the completion of the etching step to produce continuous channels from the connected segments. Thus, mask alignment errors that would cause the channels to be greatly widened when the channels are one long recess are greatly reduced.
U.S. Pat. No. 5,196,378 to Bean et al. describes a method for separating semi-conductor dice formed in a semi-conductor wafer by scribing and etching the wafer. An orientation dependent etch (ODE) or an anisotropic etch is utilized to separate the dice. The orientation dependent etch may be conducted utilizing a KOH-propanol etchant. Other anisotropic etchants include, but are not exclusive of, Tetramethyl Ammonium Hydroxide (TMAH) ethylenediamine pyrocatechol (EDP) and hydroxides of cesium and potassium as set forth in U.S. Pat. No. 4,600,934 to Anie et al.
The thermal ink jet printheads mentioned above require the formation of heater pits and bypass pits in a thick film insulative layer deposited on the heating element wafer. The bypass pits are formed in the thick film insulative layer to allow ink to pass from the reservoir to the individual channels.
The geometrical parameters and/or configurations of the ink flow paths in ink jet printheads are factors that determine the frequency of the droplet ejection and thus the printing speed. Orientation dependent etching restricts etched shapes to rectangles where fine dimensional control is required. With typical ODE etchants, convex corners are etched in a less controllable fashion. Some of the important geometrical parameters are the size of the nozzles relative to the cross-sectional area of the channels, and the size of the ink flow area at the bypass pit relative to the nozzles, for these dimensions influence capillary refill times from the ink supply in printhead reservoir. The cross-sectional area of the nozzle greatly influences latency. As fluid evaporates from the nozzle end the viscosity increases, eventually plugging the channel. Latency refers to the length of time before this plugging occurs.
Because the channels are isolated from the reservoir, the ink jet printheads require a thick insulative film with bypass pits formed therein to allow ink communication from the reservoir to the channels. Therefore, there is a need for more flexibility in the design and fabrication of silicon channel structures in ink jet printheads.
The presence of bypass and heater pits also generally results in an increase in a problem known as "dropout" caused by trapped air bubbles in the ink channels. These air bubbles get trapped in these pits, thus blocking ink flow.
Therefore, there is a need in the art for an ink jet printhead that limits the occurrence of dropout problems.
Moreover, thick insulative films formed on the heater element wafer severely limit the ink design latitude (e.g., pH, hydrolysis, viscosity, etc.). It also complicates front face wetting problems by introducing a second material (i.e., the insulation film material) on the front face of the printhead. Thus, there is a need for an ink jet printhead that allows for more flexibility in ink design latitude while limiting front face wetting problems.